Novel bidentate phosphine ligands and a process for preparing linear aldehydes by hydroformylating internal olefins using said phosphine ligands.
Linear aldehydes, particularly butyraldehyde, are of great industrial importance and, after further processing to the alcohols, are widely used in the plasticizer, solvent and polymer sector. Since mixtures of internal olefins, such as raffinate II, are produced in large amounts as a by-product in refining and cracking in the oil-processing industry, hydroformylating internal olefins to produce linear aldehydes is of great industrial interest.
The term xe2x80x9cinternal olefinsxe2x80x9d means those olefins which have at least one non-terminal double bond. However, this does not mean that internal olefins may not have a terminal double bond. Therefore, the term xe2x80x9cinternal olefinxe2x80x9d is also taken to mean, for example, a compound such as 1,3-pentadiene.
It is known to prepare aldehydes by hydroformylating olefins using a catalyst with linear and branched aldehydes generally being formed simultaneously: 
For a reaction of this type, bidentate ligands are also used as a catalyst component. In this case, for example, the ligand can be used together with a metal or in the form of a complex with a metal.
The term xe2x80x9cbidentate ligandxe2x80x9d means here and hereinafter molecules of the formula
Rxe2x95x90P-E-Pxe2x95x90R 
wherein, xe2x80x94Pxe2x95x90R and Rxe2x95x90Pxe2x80x94 are individually organic cyclic groups in which the phosphorous atoms are part of the cyclic system and are linked to the cyclic system via a phosphorus-carbon bond or a phosphorus-oxygen bond; and E is a bridging group which links the corresponding phosphorus atoms of the two organic cyclic groups.
For industrial hydroformylation reactions, a high selectivity for the linear or branched aldehydes is particularly necessary. This selectivity is generally expressed by what is termed the l/b ratio=(linear aldehyde)xc2x7(branched aldehyde)xe2x88x921. The hydroformylation is described by Frohning and Kohlpaintner in Applied Homogeneous Catalysis with Organometallic Compounds, Ed. B. Cornils, W. A. Hermann; VCH, Weinheim 1966, Vol. 1, pp. 29-104. Another example of the use of bidentate ligands in catalytic reactions is the hydrogenation described by Brunner in Applied Homogeneous Catalysis with Organometallic Compounds, Ed. B. Cornils, W. A. Hermann; VCH, Weinheim 1966, Vol. 1, pp. 201-219.
EP-0 530 015 A1 describes the use of ligands of the type Rxe2x95x90P-E-Pxe2x95x90R, such as the ligand of the formula 
which are used in metal catalysts for the chiral synthesis of pharmaceuticals and novel intermediates. JP 07082281 A2 (JP 93-225998) discloses that ligands of this structural type can be used in hydroformylation for the synthesis of branched olefins with high selectivity.
Hopps describes in J. Organ. Chem., 1981, Vol 46, pp. 4422-4427, the use of a ligand of the type Rxe2x95x90P-E-Pxe2x95x90R, such as the ligand of the formula 
for the asymmetric hydroformylation of vinyl acetate, vinyl propionate and vinyl benzoate, the selectivity for the branched aldehydes being 75-95%.
EP-0 213 639 B1 describes a bidentate phosphite ligand of the type Rxe2x95x90P-E-Pxe2x95x90R, where Pxe2x95x90R or Rxe2x95x90P are individually organic cyclic groups in which the phosphorus atoms are part of the cyclic system and are linked to the cyclic system via a phosphorus-oxygen bond, and E is a bridging group which links the two phosphorus atoms of the two organic cyclic groups and where the phosphorus atom is linked to the bridging group E via a phosphorus-oxygen bond. These ligands are, for example, a ligand of the formula 
and can be used for hydroformylating internal olefins to produce linear aldehydes. The multistage synthesis of this ligand and the lower stability of phosphite ligands compared with phosphine ligands in general is, however, a disadvantage for industrial implementation.
It is an object of the invention to provide novel ligands and a process for hydroformylating internal olefins to produce linear aldehydes, which overcomes the disadvantages of the processes described for hydroformylating internal olefins and which converts internal olefins to produce linear aldehydes with high selectivity.
These and other objects of the invention will become obvious from the following detailed description.
The novel ligands of the invention are bidentate phosphine ligands of the formula 
wherein R1, R2, R3 and R4 are independently selected from the group consisting of hydrogen, fluorine, alkyl of 1 to 8 carbon atoms, alkoxy of 1 to 8 carbon atoms, acyloxy of an organic carboxylic acid of 1 to 8 carbon atoms, aryl of 6 to 18 carbon atoms, aryloxy of 6 to 18 carbon atoms, xe2x80x94CN, xe2x80x94CF3, xe2x80x94CHO, xe2x80x94SO3H, xe2x80x94SO3M, xe2x80x94SO2R, xe2x80x94SOR, xe2x80x94NH2, xe2x80x94NH-alkyl of 1 to 8 carbon atoms, xe2x80x94N-alkyl2 of 1 to 8 carbon atoms, xe2x80x94NHCO-alkyl, xe2x80x94N-(alkyl)xe2x80x94(CO-(alkyl) where the alkyl have 1 to 4 carbon atoms, xe2x80x94COO-alkyl of 1 to 8 carbon atoms, xe2x80x94CONH2, xe2x80x94CO-alkyl of 1 to 8 carbon atoms, xe2x80x94NHCOH, xe2x80x94NHCOO-alkyl of 1 to 4 carbon atoms, xe2x80x94CO-aryl of 1 to 8 carbon atoms, xe2x80x94COO-aryl of 1 to 8 carbon atoms, xe2x80x94CHCHxe2x80x94CO2-alkyl of 1 to 8 carbon atoms, xe2x80x94PO-(-aryl)2 of 1 to 8 carbon atoms, xe2x80x94PO-(alkyl2) of 1 to 4 carbon atoms; M is a cation selected from the group consisting of alkali metal ions, alkaline earth metal ions, xe2x80x94NR2H2, xe2x80x94NR3H, xe2x80x94NRH3, xe2x80x94NR4, xe2x80x94NH4, xe2x80x94PR2H2, xe2x80x94PR3H, xe2x80x94PRH3, xe2x80x94PR4 and xe2x80x94PH4; or R1, R2, R3 and R4, with one another, together form at least one aliphatic or aromatic ring of 5 to 20 carbon atoms; E is a bridge linking the two phosphorus atoms, where the number of atoms situated between the two phosphorus atoms is 2 and 6, selected from the group consisting of C, N, Si, S, O, P, Fe and As; X is selected from the group consisting of xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94Si(Ra)2xe2x80x94, xe2x80x94Si(ORa)2xe2x80x94, xe2x80x94N(C(O)Ra)xe2x80x94, xe2x80x94N(Rb)xe2x80x94, xe2x80x94C(Rc)(Rc)xe2x80x94, xe2x80x94C(O)xe2x80x94, xe2x80x94N(SiRd)xe2x80x94, xe2x80x94P(Rd)xe2x80x94, xe2x80x94P(O)(Rd)xe2x80x94, xe2x80x94Cxe2x95x90C(Rc)(Rc)xe2x80x94 and xe2x80x94P(ORd)xe2x80x94 wherein
Ra is alkyl of 1 to 8 carbon atoms,
Rb is aryl of 6 to 18 carbon atoms,
Rc is selected from the group consisting of hydrogen, alkyl of 1 to 8 carbon atoms, aryl of 6 to 18 carbon atoms, alkoxy of 1 to 8 carbon atoms, aryloxy of 6 to 18 carbon atoms, Ra(O)xe2x80x94 and Rb(O); and
Rd is one of Ra or Rb.
According to a preferred embodiment, E is one of the following groups: 
wherein X is selected from the group consisting of xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94Si(Ra)2xe2x80x94, xe2x80x94Si(ORa)2xe2x80x94, xe2x80x94N(C(O)Ra)xe2x80x94, xe2x80x94N(Rb)xe2x80x94, xe2x80x94C(Rc)(Rc)xe2x80x94, xe2x80x94C(O)xe2x80x94, xe2x80x94N(SiRd)xe2x80x94, xe2x80x94P(Rd)xe2x80x94, xe2x80x94P(O)(Rd)xe2x80x94, xe2x80x94Cxe2x95x90C(Rc)(Rc)xe2x80x94 and xe2x80x94P(ORd)xe2x80x94,
Ra is alkyl of 1 to 8 carbon atoms
Rb is aryl of 6 to 18 carbon atoms
Rc is selected from the group consisting of hydrogen, alkyl of 1 to 8 carbon atoms, aryl of 6 to 18 carbon atoms, alkoxy of 1 to 8 carbon atoms, aryloxy of 6 to 18 carbon atoms, Ra(O)xe2x80x94 or Rb(O)xe2x80x94; and Rd is one of Ra or Rb;
Y is oxygen or sulfur; and
R5s are individually aryl of 6 to 18 carbon atoms or alkyl of 1 to 8 carbon atoms.
In accordance with a further preferred embodiment of the invention, E is one of the following groups: 
where R6 is alkyl of 1 to 8 carbon atoms or aryl of 6 to 18 carbon atoms; Z is oxygen or nitrogen, and n is an integer of 2 to 6.
The phosphine ligands of the invention are used, in particular, in a process for preparing linear aldehydes by hydroformylating internal olefins of 4 to 12 carbon atoms in the presence of a bidentate phosphine ligand of the formula 
wherein R1, R2, R3, R4, M, E and X are defined as above.
According to a preferred embodiment of the process of the invention, E is one of the following groups: 
wherein X, Y, R5 and R6 are defined as above.
In accordance with a further preferred embodiment of the process, E is one of the following groups: 
where R6 is an alkyl of 1 to 8 carbon atoms or aryl of 6 to 18 carbon atoms; Z is oxygen or nitrogen; and n is an integer of 2 to 6.
It has proved to be particularly expedient if the reaction is carried out in the presence of rhodium at a concentration of from 1 to 1000 ppm, preferably from 10 to 250 ppm, based on the total reaction mixture. The ratio of rhodium to ligand can, in this case, be between 1:1 and 1:100, preferably between 1:1 and 1:20.
The temperature during the reaction is generally between 10 and 180xc2x0 C., preferably between 80 and 140xc2x0 C., and the pressure is between 0.1 and 200 bar, preferably between 1 and 100 bar.
The reaction can be carried out in the presence of a solvent which may be selected from the group consisting of ether, CO2, fluorinated hydrocarbons, toluene and benzene. However, the solvent can also be a polar aprotic solvent which is preferably selected from the group consisting of DMAC, DMF or NMP.
It is also possible to carry out the reaction in the presence of an oligomeric linear aldehyde, preferably particular in the presence of the trimer of the linear aldehyde to be prepared, which here also acts as solvent. Also, it has proved to be expedient to carry out the reaction in a two-phase mixture of the solvent and water.
The CO/H2 ratio during the hydroformylation of the invention is usually between 1:10 and 10:1, preferably between 1:2 and 2:1.
In the following examples, there are described preferred embodiments to illustrate the invention. However, it should be understood that the invention is not intended to be limited to the specific embodiments.